(Bloomberg) -- One of the strongest earthquakes among many in Oklahoma since the introduction of hydraulic fracturing struck early Saturday near a complex of oil-storage facilities, leading state regulators to order the suspension of about 37 wastewater-disposal wells.

The Oklahoma Corporation Commission, which regulates the state’s oil industry, is contacting the operators of the wells in a 500-square-mile area around the town of Pawnee, Governor Mary Fallin said in a Twitter post. Oil storage and pipeline facilities at Cushing, Oklahoma, 25 miles (40 kilometers) south of Pawnee, were undamaged, according to the commission and four of the companies that operate there.

Matched strongest quake ever in state

The 5.6-magnitude quake matched a November 2011 tremblor as the strongest ever in the state, the U.S. Geological Survey said, and was followed by at least eight others measuring as much as 3.6. The surrounding region of Oklahoma and Kansas, a center of oil exploration using fracking, has had almost 80 quakes of magnitude-4 or higher over the past decade.

Oklahoma, a region not known for seismic activity, began having earthquakes in 2009, the same year area oil companies began using fracking to shatter deep rock layers to extract oil and gas. Fracked wells produce large quantities of wastewater, which drilling companies inject into ultra-deep disposal wells. Critics blame earthquakes on this practice.